Cannot truly separate parts
The objects in the file you provided in the comments of your question are linked copies.

Select them all together;
press F3 (or spacebar, if in blender 2,79 or earlier);
type &quot;make single user&quot; to find the &quot;Make Single User&quot; command and click it;
in the popup that appears check &quot;Object Data&quot; (see img) and press OK.